In pelvic operations, in particular in hip replacement operations, patients typically are operated on while lying on their side, wherein it may be necessary to fix the patient's pelvis such that it is immobile. This can be achieved using patient positioners that include support devices for the lower region of the patient's body. In this position, however, it is difficult to ascertain navigation information by tapping body landmarks (ASIS and pubic points) within the framework of medical navigation. This is due to the fact that these characteristic pelvic landmarks are in most cases almost inaccessible, at least in the lower region. X-ray or fluoroscopic registration are therefore often used.
Many of the conventional patient positioners use cushions or rods that are fixed to an arm system of the operating table or to similar fixed means. Such a mechanical positioner, for example, is known from U.S. Pat. No. 6,311,349.
In addition to mechanical positioners, there are also positioners that include navigation reference means for medical navigation; such a positioner, for example, is known from WO 2004/089 192 A2. The reference means can be used to indirectly localize the ASIS and pubic points and, therefore, to register the front pelvic plane.
A problem with existing positioning means is seen when fluoroscopic images are recorded. It is within the nature of the positioning means that at least a part of their support devices lie in the radiation path of the fluoroscopic recording and, therefore, can create artefacts on the recording. Even if the chosen materials for the support device are permeable to x-ray radiation, shadows and edges may still be created on the images. These artefact contours superpose the anatomical structure (pelvis) and could result in a misinterpretation of the recordings. The navigation software, for example, detect anatomical structures in order to register the pelvis, and the above-mentioned shadows and edges could in this case lead to an incorrect choice of contour and, thus, to incorrect registration. In addition, it is difficult to define points in the region of the support images because the contrast such regions suffers due to the artefacts.